Known embedded pole parts are usually integrated in a medium voltage to high voltage circuit breaker. Medium voltage circuit breakers are rated between 1 kV and 72 kV of a high current level. These specific circuit breakers interrupt the current by creating and extinguishing the arc in a vacuum container. A pair of corresponding electrical switching contacts is accommodated inside the vacuum container. Modern vacuum circuit breakers attend to have a longer life expectancy than former air circuit breakers. Although, vacuum circuit breakers replace air circuit breakers, the present disclosure is not only applicable to vacuum circuit breakers but also for air circuit breakers or modern SF6 circuit breakers having a chamber filled with sulfur hexafluoride gas instead of vacuum.
The document EP 2 278 601 A1 discloses an embedded pole part with an isolating housing made of thermoplastic material, which accommodates a vacuum interrupter as well as electric terminals wherein at the outer surface of the housing horizontal and/or vertical aligned 3-dimensional structures joined by material engagement are implemented into the thermoplastic material, in order to achieve a higher mechanical stiffness as well as higher creepage length of the embedded pole part.
The embedding of vacuum interrupters in epoxy material is a well-tested technology and in this technique the filling pressure is low and it will not cause damage of the vacuum interrupter. Furthermore, the force on the electric terminal is also not critical and no special fixation is needed, but the filling time and curing time are relatively long. Injection moulding of thermoplastic material is also used in this field of technology. During the injection moulding process, the pressure in the cavity of the mould is very high during the filling and packing period. By using injective moulding method with thermoplastic material instead of epoxy material to embed the vacuum interrupter inside the insulation material, the difference is the pressure value applied to the insert. At reactive epoxy moulding situation the pressure can be from several bars to maximum 20-30 bars.
In injection moulding for vacuum interrupters, the maximum pressure could reach several hundred bars. When considering the long-term stability of thermoplastic material, the water affinity (the water up-take) of the thermoplastic material should be taken into account.
Known embedded pole parts which are made by epoxy material are filled with aluminum oxide or silica based on silica dioxide as filler material with a percentage of 50 wt.-%. to 70 wt.-%. The rest of the injected embedding material is the epoxy material to wet the filler material. The quantity of the filler material cannot be increased because the viscosity of the injected embedding material increases too, so that the injected embedding material could not flow through the pumping and the pipe system. Therefore, the molding to produce the epoxy part such as for the embedded pole part that cannot be sufficiently filled. Another aspect is the mechanical property of the produced part. The standard powder like silica particles as well as the fused silica particles have sharp edges so that under mechanical or dielectric load the embedded pole part is limited in these both properties.
It is a key-condition, for embedded pole parts to strengthen the pole part mechanically in such a way, that it is strong enough to withstand the short circuit current. Furthermore, it should have enough mechanical withstand, to fix the vacuum interrupter in the circuit breaker during mechanical stress if it is switched. Under these conditions, it is also important to care for dielectric stability.